An accurate ab initio HOCl potential energy surface, vibrational and rotational calculations, and comparison with experiment

Accurate ab initio multireference configuration interaction ~CI! calculations with large correlation-consistent basis sets are performed for HOCl. After extrapolation to the complete basis set limit, the ab initio data are precisely fit to give a semiglobal three-dimensional potential energy surface to describe HOCl→Cl1OH from high overtone excitation of the OH-stretch. The average absolute deviation between the ab initio and fitted energies is 4.2 cm for energies up to 60 kcal/mol relative to the HOCl minimum. Vibrational energies of HOCl including the six overtones of the OH-stretch are computed using a vibrational-Cl method on the fitted potential and also on a slightly adjusted potential. Near-spectroscopic accuracy is obtained using the adjusted potential; the average absolute deviation between theory and experiment for 19 experimentally reported states is 4.8 cm. Very good agreement with experiment is also obtained for numerous rotational energies for the ground vibrational state, the ClO-stretch fundamental, and the fifth overtone of the OH-stretch. © 1998 American Institute of Physics. @S0021-9606~98!00231-1#